In 1972, the Yakovlevdesign bureau started work on a short to medium range airliner capable of carrying 100–120 passengers. It was intended to be a replacement for the Tupolev Tu-134 jet as well as the Ilyushin Il-18, Antonov An-24 and An-26 turboprop airliners. While the new airliner was required to operate out relatively small airfields while maintaining good economy, as many Soviet airports had been upgraded to accommodate more advanced aircraft, it did not have to have the same ability to operate from grass strips as the Yakovlev's smaller Yak-40. The requirement resulted in the largest, heaviest and most powerful aircraft designed by Yakovlev so far.[3][4]

Initial design proposals included a straight-wing airliner powered by two Soloviev D-30turbofans and resembling an enlarged Yak-40, but this was rejected as it was considered uncompetitive compared to Western airliners powered by high bypass ratio turbofans. Yakovlev settled on a design powered by three of the new Lotarev D-36three-shaft high bypass turbofans, which were to provide 63.90 kN (14,330 lbf) of thrust. Unlike the Yak-40, the new airliner would have swept wings.[4][5]

The Yak-42 is a low-winged monoplane of all-metal construction, with a design lifespan of 30,000 one-hour flights.[6] It has a pressurised fuselage of circular section, with the cabin designed to carry 120 passengers in six abreast layout (or 100 passengers for local services with greater space allocated to carry-on luggage and coat stowage). The aircraft is flown by a flight crew of two pilots sitting side-by-side in a flight deck forward of the cabin. Access is via two airstairs, one in the underside of the rear fuselage, like that of the Yak-40, and one forward of the cabin on the port side. Two holds are located under the cabin, carrying baggage, cargo and mail.[7]

The wing layout underwent considerable revision during the design process, with the first prototype being built with a wing sweep of 11 degrees and the second prototype with a sweep of 23 degrees. After evaluation, the greater sweep of the second prototype was chosen for production. Early aircraft had a clean wing leading edge with no control surfaces, and plain trailing edge flaps. This changed in later aircraft, which were fitted with leading edge slats, with the trailing edge slats slotted.[6][7][8]

Two engines were mounted in pods on either side of the rear fuselage, with the third embedded inside the rear fuselage, fed with air via an "S-duct" air inlet. An auxiliary power unit (APU) is also fitted in the rear fuselage. No thrust reversers are fitted. The aircraft has a T-tail, with both the vertical fin and the horizontal surfaces swept.[7]

The first of three prototypes, which was fitted with an 11-degree wing and registeredCCCP-1974, made its maiden flight on 7 March 1975. It was followed by the second prototype, (CCCP-1975) with the 23-degree wing and a cabin with 20 rows of windows instead of 17 in the first prototype, and a third prototype (CCCP-1976) fitted with improved de-icing gear.[8][9]

The first production aircraft was completed on 28 April 1978, with the first scheduled passenger flight, on the Aeroflot Moscow-Krasnodar route taking place on 22 December 1980. Production was at first slow, with only 10 flown by mid-1981. Initial aircraft were fitted for 120 seats in a three-plus-three arrangement. This was soon changed to a first class section with two-plus-two seating, and a main cabin with ninety six seats, giving a total of 104 seats.[10][11]

In its first year of operation Aeroflot's Yak-42s carried about 200,000 passengers, mainly on routes from Moscow, but also on international services from Leningrad to Helsinki and from Donetsk to Prague, with the type being planned to enter wider service throughout the Aeroflot fleet.[12] On 28 June 1982, however, the tailplane detached from an Aeroflot Yak-42 in flight owing to a failure of the actuator screw jack, causing the aircraft to crash fatally near Mazyr. The type was grounded as a result, not returning to service until October 1984.[13]

An export order for seven aircraft was announced in 1982 by Aviogenex of Yugoslavia, but the contract lapsed.[11] The availability of the longer-range Yak-42D variant from 1991 onwards gave rise to a few more export sales, to Cuba and China.[14] As of 1 January 1995 a total of 185 Yak-42 had been produced, including 105 Yak-42D.[1]

Conversion of a Yak-42 for geophysical survey and environmental monitoring. Fitted with large underwing pods containing electro-optical sensors.[17]

Yak-42LL

Conversion as testbed for Progress D-236propfan engine. Single D-236 (rated at 8,090 kW (10,850shp)) mounted in place of starboard engine, on special pylon to give sufficient clearance for 4.2 m (13 ft 9¾ in) propellers. First flew 15 March 1991.[1]

Yak-42M

A projected but unbuilt stretched airliner. Planned to be powered by three Progress D-436 turbofans, a stretched fuselage and new wings. Design developed into Yak-242.[18]

Flight Leningrad-Kiev, damage to stabilizer due to mechanical deterioration, diving and disintegrating in mid-air. All Yak-42 flights were suspended until the design error was fixed. This was the deadliest Yak-42 crash, and the deadliest air crash in Belarus.